This invention relates generally to monitoring the internal temperature of an electronic device; and, more particularly, relates to a system and method for detecting and recording the temperature of an implantable medical device, such as a pacemaker or defibrillator, and using the temperature information to control the operation of the implantable medical device.
Temperature measurements have been utilized in the past in conjunction with the control and operation of surgical medical tools. Such measurements are commonly used to determine when a treatment using heating energy is completed. For example, surgical tools are available to deliver energy to tissue at a point of incision to cause coagulation and prevent excessive bleeding. Such surgical tools may use temperature measurements taken from the treated tissue to determine when coagulation has been completed. A system of this nature is disclosed by U.S. Pat. No. 5,707,369 to Vaitekunas et al., which describes the use of temperature to determine when coagulation of tissue has occurred to a desired degree. Another similar example is provided by U.S. Pat. No. 5,496,312 to Klicek, which describes use of temperature measurements to regulate power supplied by a electrosurgical generator used to perform tissue desiccation. Tissue ablation systems also commonly use temperature measurements of adjacent tissue to control the amount of delivered ablation energy. Examples of these types of systems are provided by U.S. Pat. Nos. 5,743,903 and 5,906,614, both to Stern et al., and U.S. Pat. No. 5,810,802 to Panescu et al.
In addition to measuring temperature of adjacent body temperature, some medical systems use the temperature of internal circuitry to adjust system operation and performance. U.S. Pat. No. 5,897,576 to Olson et al. discloses an external defibrillation system that senses the temperature inside the power supply case of the device, and uses these measurements to adjust the operating parameters of the system, including capacitor charge time.
Implantable Medical Devices (IMDs) have also been provided that utilize temperature measurements to adjust therapy delivery. U.S. Pat. No. 4,905,697 to Heggs et al. describes a system that uses temperature sensed within the right ventricle of the heart to determine when a patient is exercising so that pacing rate may be elevated. Another similar system is disclosed in U.S. Pat. No. 4,782,836 to Alt et al. U.S. Pat. No. 5,814,087 to Renirie discloses a system that uses a drop in blood temperature to detect the onset of sleep so that pacing rate may be adjusted accordingly. Other pacemaker systems have been provided that use body temperature to perform capture detection. Such a system is disclosed in U.S. Pat. No. 5,336,244 to Weijand. Yet another example of the use of temperature within an IMD is provided by U.S. Pat. No. 5,089,019 to Grandjean, which discloses a system that uses intramuscular temperature to monitor performance of an implantable skeletal muscle powered cardiac assist system.
In addition to using temperature measurements for the purposes discussed in the foregoing paragraphs, a need exists to utilize temperature to regulate and control the system functions of an IMD that are not directly related to therapy delivery. One case in which temperature may be utilized in such a manner involves what is known as an Elective Replacement Indicator (ERI). An ERI is an indicator that is asserted when the battery power of an IMD has reached a predetermined low level. Upon assertion of this indicator, a battery replacement procedure is typically scheduled. This indicator may also be used to disable non-essential circuits so that overall power consumption decreases and battery power is conserved. The assertion of the ERI may further cause a pacing device to revert to a nominal pacing mode to further conserve power. One problem with the ERI is that sometimes this indicator may be erroneously set. This may occur prior to implant when an IMD is subjected to cold temperatures for a period of time. Cold temperatures cause the voltage of the IMD battery to drop. This initial voltage drop, or the subsequent voltage increase that occurs when the IMD returns to a warmer temperature, may cause the low-battery condition to be incorrectly detected. This, in turn, results in a latching of the ERI condition. In some circumstances, this may place the IMD in a low-battery state that can only be corrected using a manual override. To prevent this erroneous detection of a low-power condition, it is desirable to disable the ERI when the IMD reaches a predetermined low temperature.
Another situation in which temperature measurements may be used to control the logic functions of an IMD relates to a high-temperature quality control process known as xe2x80x9cburn-inxe2x80x9d. The burn-in process is used during manufacturing to stress the electrical components of the pacemaker by subjecting them to high temperatures and high operating voltages. The components that fail during burn-in are not used in products. Although most circuits are required to function at a burn-in temperature of 135xc2x0 C., some circuits to such as CMOS bias generator mirror structures experience a significant shift in operating point that affects testing results. Additionally, some analog circuits must be provided with an increased current to operate properly at elevated temperatures. It is desirable to sense burn-in temperatures so that functions vulnerable to the high stress factors can be automatically disabled or appropriately compensated, and burn-in test results will not be erroneously affected by these circuits.
Yet another need for using temperature to control IMD operation involves the disabling of pacing and other functions of the IMD during the shelf-life of the device. This is useful for conserving battery power until the time of implant. This disabling function could be triggered, for example, by determining that the temperature of the IMD is not being maintained at a temperature that generally coincides with body temperature. Similarly, the detection of a temperature approximating body temperature could be used by the IMD to confirm the occurrence of implantation. These measurements could be used either alone, or in conjunction with other detection mechanisms such as body activity or impedance measurements taken between electrodes, to detect implantation.
Other uses of IMD temperature involve obtaining temperature measurements to control other treatment-related functions of the medical device. For example, the IMD temperature may be recorded to detect periods of extended exercise. Such extended periods of exercise will generally result in raising the internal body temperature. Upon detection of extended periods of exercise, the decay time constants can be adjusted. For example, following a prolonged period of exercise, it may be desirable to increase the time over which an elevated pacing rate is slowed to a normal pacing rate. It may also be useful to use temperature to set upper and lower rate limits. For example, after implantation is detected, a temperature above body temperature that is sustained for an extended period may be used to limit the upper pacing rate. Similarly, following implant, if a temperature below body temperature is detected for a substantial period to time as may occur during medical procedures such as an ice bath, the lower pacing rate may be limited so that the pacing rate does not drop too low.
Temperature measurements may also be used to compensate logical functions that are temperature sensitive. For example, some circuits such as Analog-to-Digital Converters require a reference voltage to operate properly. However, the reference voltage may drift with temperature changes, causing circuit operation that also varies with the temperature. A voltage compensation circuit may be provided to compensate for reference variations that are the result of temperature changes. Such a device may be programmably re-calibrated with data that reflects a current temperature, thereby making the circuit operation relatively temperature independent.
It is thus an object of the invention to provide an integrated temperature sensing circuit in an IMD to control system functions. Such control may prevent the erroneous assertion of an ERI indicator during exposure to cold temperatures, while allowing for correct burn-in results during high-temperature operations. This control may further provide a verification of implantation, or may be used to monitor a patient""s internal temperature for immediate diagnostic purposes and to establish long-term trends in patient health. Temperature measurements may provide an indication of the duration of exercise so that decay constants may be adjusted, and detect external conditions such as medical procedures that require change of system functions. Temperature changes may also cause the recalibration of certain circuit functions so that circuit operation can be made temperature independent.
The current invention is an Implantable Medical Device (IMD) having a temperature sensor to sense the temperature of the IMD, and to utilize the temperature measurements to control at least one system function provided by the IMD. Generally, this control relates to determining or controlling the state of the IMD. In one embodiment, temperature measurements are utilized to prevent the erroneous latching of a low-power warning indication that may be falsely triggered if an IMD is subjected to cold conditions during storage prior to implant. In another embodiment, the temperature measurements are used to detect the occurrence of a burn-in process that is used to stress IMDs during testing. If burn-in conditions are detected, it may be desirable to automatically disable certain ones of the analog circuits. This places certain circuits in a known state, and may further compensate voltage and current levels for some circuits so that the burn-in tests can be completed normally without the testing system requiring manual intervention. Another aspect of the invention uses temperature readings to allow temperature-sensitive circuits to be programmably recalibrated if temperature changes occur. Yet another embodiment of the invention uses temperature measurements to confirm the occurrence of implantation of a device. If implantation has not yet occurred, certain logical functions may be disabled to conserve battery power.
Other embodiments of the invention may utilize temperature measurements to monitor patient environment and activity levels. For example, extended periods of elevated temperature measurements may be used to detect prolonged periods of exercise. Such situations may prompt the extension of decay times so that pacing rates decay more gradually from an elevated rate to a resting rate following termination of the patient activity. In other situations, elevated temperatures may prompt the setting of an upper pacing rate limit. This may be desirable if the temperature increase is caused by a non-exercise condition such as the exposure to an X-ray source. This type of state could be detected by using measurements from both a temperature sensor and an activity sensor, for example. Yet other situations involving an extended period of colder-than-normal temperatures detected following implant may prompt the setting of a lower pacing rate limit so that pacing rates do not drop excessively during treatments such as ice baths. Temperature measurements are also available to be transferred to external devices such as programmers for later diagnostic purposes.
In one embodiment of the invention, the temperature sensor of the IMD comprises two mismatched transistors connected to a current supply with the transistors having substantially the same collector current. The transistors have a voltage difference between their control nodes that is proportional to the temperature of the electronic device. The mismatched transistors may be bipolar transistors having a size ratio of eight to one. The IMD may further comprise a gain circuit to programmably set the scale and resolution of the temperature sensor.